1. Field of the Invention
The present invention relates generally to the field of variable geometry turbine design and, more particularly, to method and system for controlling the vane position of a variable geometry turbocharger.
2. Related Art
Turbochargers are frequently utilized to increase the output of an internal combustion engine. A typical turbocharger comprises a turbine wheel coupled to a compressor wheel by a common shaft. Exhaust gas from the engine diverted into the turbocharger through an inlet nozzle spins the turbine wheel, which in turn spins the shaft and the compressor wheel. The spinning compressor wheel is able to force ambient air into the engine combustion chambers at a higher pressure than the engine can otherwise aspirate, resulting in what is commonly referred to as xe2x80x9cboost pressure.xe2x80x9d In this manner, a larger air mass and fuel mixture is achieved in the engine, which translates to greater engine output during combustion. The gain in engine output is directly proportional to the increase in air flow generated by the turbocharger boost pressure. However, allowing the boost pressure to reach too high a level can result in severe damage to both the turbocharger and the engine, particularly when the engine has to operate beyond its intended performance range.
One of the primary objectives of turbocharger design is therefore to regulate boost pressure so as to optimize power output at different engine operating conditions without causing engine damage. A known technique for regulating boost pressure is the variable geometry turbocharger (xe2x80x9cVGTxe2x80x9d) design. Two such VGT designs are the Garrett(copyright) VNT(trademark) and AVNT(trademark) designs, which use multiple adjustable vanes to control the flow of exhaust across the turbine wheel. Placed at the inlet nozzle, the vanes can be opened incrementally wider to permit greater gas flow across the turbine wheel, causing the turbine wheel to spin at a slower speed and lowering the boost pressure. Alternatively, the vanes can be closed incrementally narrower to raise the boost pressure. Thus, the amount of boost pressure generated by the turbocharger can be regulated by varying the vane position so as to optimize engine output, whether for fuel economy, emission, or response, while avoiding engine or turbocharger damage.
Thus, there is an intense need in the art for system and method for controlling vane position of a variable geometry turbocharger.
The present invention discloses control method for variable geometry turbocharger and related system. According to the present invention, a boost target is determined for a turbocharger. For example, the boost target can be determined from a boost target map that contains desired boost pressure for different measured engine parameters such as engine speed and/or fuel quantity. The boost target may also be corrected based on the ambient pressure to prevent overspeeding the turbocharger in instances where the ambient pressure is too low, for example.
Next, an error value between the target boost and the actual boost pressure is calculated. The error value is then utilized to determine a new vane position that is needed in order to realize the boost target. In one embodiment, the new vane position is determined by first calculating a required change in vane position according to the equation, xcex94xcex8=kp(err)+kdxc2x7d(err)/dt, where xcex94xcex8 is the change in vane position, kp is a proportional gain value, kd is a differential gain value, and err is the error value between the boost target and the actual boost. Following, xcex94xcex8 is summed with the xe2x80x9coldxe2x80x9d or previous vane position to determine the desired new vane position. Subsequently, the turbocharger""s set of vanes is positioned according to the new vane position determined previously. The vanes can be positioned by an actuator, for example, that is driven by a signal from a digital-to-analog converter (D/A) configured to convert the new vane position into an analog signal.
In one disclosed embodiment, an open loop diagnostic mechanism can be employed, prior to positioning the turbocharger""s vanes, wherein a target vane position is determined from an open loop map. Following, the difference in value between the new vane position and the target vane position is calculated and compared to a threshold fault value. If the difference in value is equal to, or exceeds the threshold fault value, an error signal can be sent to an engine control module to enter fault mode.
In one disclosed embodiment, a feed forward mechanism is utilized to enhance control prior to positioning the turbocharger""s set of vanes. In one disclosed embodiment, a control system is assembled to realize the processes set forth above.